I'm suprised you didn't notice issues with backing the boat. I got a similar encrustation at the end of last season, just on my prop. The first symptom noticed was the boat wouldn't back up worth a hoot. That alone didn't cause the penny to drop, but when the performance going forward was not as expected, I immediately suspected a prop load of barnacles, ony because my slip neighbor had told me the same thing happened to him the year before when I had no problem at all on the other side of the marina.
s/v Palmetto Moon
1991 Catalina 36